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I know the sponge may be better at mechanical filtration, and the rings may be better as far as surface area and pores, but what other differences are there between the two? Are there any situations where one can be used in place of another, or are they too different either due to something I don't know about, or simply the differences I mentioned first?

TIA.

One more question, where would say lava rock fit in with all of this, or say pot scrubbers? (The round kind)

The name of the game is surface area. In a canister filter, we have limited space for media so the more surface area the bio media has, the more effective it is.

I found website somewhere (I'll try and dig up the link) with the amount of surface area for each media.

The ceramic / spun glass media has a LOT more surface area than the next closest media. This is by far the best media you can put in your filter for bio filtration. The issue with this media is the price... it's the most expensive. However with sites like aquatraders.com and ebay selling generic bio rings ( 1L container for $7 USD) the price isn't really much of a factor. This becomes a no-brainer. Get the generic media and get the most surface area for the best price.

The lava rock is less expensive, but it's also a lot less surface area. Lava rock is a good choice if you have a very large DIY type of filter where you can have a whole lot of lava rock to compensate for the surface area. In a canister filter where space is limited... the rings / nuggets / stars are still a better choice.

Pot scrubbies and sponges suck in my opinion. Here's why... 1. Surface area is very low when compared to the ceramic media. 2. Cleaning.
No matter how good the mechanical media is in your canister, some gunk will make it through and get onto the bio media. You do need to rinse your bio media on occasion. Even by rinsing in tank water (which is silly) you're still going to remove most of if not all of the nitrifying bacteria when you clean these media types (sponges and scrubbies). Comparitively when you rinse a ceramic nugget, they rinse clean very quickly and the interior pores of the media are still full of bacteria.

Scrubbies and sponges can work (the bacteria will grow on ALL surface area in the tank / filter), but if you have a couple of dollars in your wallet you're much better off with the rings / nuggets / stars. The brand of these doesn't really matter. The only real differences are shape, packaging, and price. Cheapest is best IMO.

It's going to cost about thirty bucks to set it up as suggested here. I already have a bio-sponge, bag of carbon, and bio rings, I was hoping to finish it off with something like pot scrubbers instead of two more bags of bio-rings and another bio-sponge. Replacing both with pot scrubbers or lava rocks works price wise, biologically I'm not so sure.

I can fill an ENTIRE FX5 with media for $40. If you have most of the media, it shouldn't cost nearly $30 to top off the bio media. What kind of canister do you have?

Here's 4L of bio media for $20. (unless you have an empty FX5, or the large eheim classic you won't be able to fit 4L of bio media)
http://www.ebay.com/itm/4x-1L-Odyssea-G ... 0223250752

Here's 1L of bio media for $7
http://www.aquatraders.com/Odyssea-Supe ... /46013.htm

Here's Purigen in a media bag for $8
http://www.bigalspets.com/Seachem-Purig ... =salesrank

If you want carbon (not always needed if you use Purigen), here's a really good deal:
1.5lbs of carbon for $11
http://www.bulkreefsupply.com/store/1-5 ... arbon.html